Electric power steering apparatus control apparatus

ABSTRACT

In the electric power steering apparatus which controls a motor that gives a steering assisting force to a steering mechanism based on an electric current controlling value which is computed from a steering assisting command value which has been computed by a computing device based on a steering torque generated in a steering shaft and an electric current value of the motor, provided are a self-aligning torque estimating section which estimates a self-aligning torque by a disturbance observer constitution and a steering torque feedback section which performs definition of a steering reaction force based on a self-aligning torque estimated value which has been estimated by the self-aligning torque estimating section and feeds the steering reaction force back to the steering torque.

TECHNICAL FIELD

The present invention relates to a power steering apparatus for steeringa wheel for steering of an automotive vehicle in accordance with adriver's operation and particularly to an electric power steeringapparatus which can independently design treatments of, for example,road surface information and disturbance information and steering safetyand also can obtain a safe, comfortable steering performance which iseasily tunable.

BACKGROUND ART

Steering of an automotive vehicle is performed by transmitting anoperation (ordinarily a rotation operation of a steering wheel) of asteering device provided inside a vehicle compartment to a steeringmechanism provided outside the vehicle compartment for performing aturning maneuver in the direction of a wheel (ordinarily, front wheel)for steering.

As for such steering mechanisms for automotive vehicles, various typesof steering mechanisms such as a ball screw type and a rack-pinion typehave been in practical use. For example, the steering mechanism of therack-pinion type, which is configured such that sliding in an axialdirection of a rack shaft extended in a right-and-left direction at afront portion of a vehicle body is transmitted to each of right and leftfront wheels via a tie-rod and a knuckle arm provided thereto, isconstituted such that a pinion, which is fit into a tip end of arotation shaft (steering column) of the steering wheel extending tooutside the vehicle compartment, is meshed with a rack gear formed in amiddle section of the rack shaft and, then, the rotation of the steeringwheel is converted into sliding in an axial direction of the rack shaft,to thereby perform steering in accordance with a rotation operation ofthe steering wheel.

Further, in recent years, a power steering apparatus, which isconstituted such that an actuator for a steering assistance such as ahydraulic cylinder or an electric motor is provided in a middle sectionof the steering mechanism, the actuator is driven in accordance with adetection result of a steering force to be added to the steering wheelfor steering, a movement (drive) of the steering mechanism in accordancewith the rotation of the steering wheel is assisted by an output fromthe actuator and, then, a labor load of a driver is alleviated, haswidely been applied.

Now, an ordinary constitution of the electric power steering apparatusis described with reference to FIG. 8. A shaft 2 of a steering wheel 1is connected with a tie-rod 6 of a direction-maneuvering wheel via areduction gear 3, universal joints 4 a and 4 b, and a pinion-rackmechanism 5. A torque sensor 10 for detecting a steering torque isprovided to the shaft 2. A motor 20 for assisting the steering force ofthe steering wheel 1 is connected to the shaft 2 via the reduction gear3. A control unit 30 for controlling the power steering apparatus, whichis supplied with an electric power from a battery 14 via an ignition key11 and a relay 13, computes a steering assisting command value I of anassisting command based on a steering torque T detected by the torquesensor 10 and a vehicle speed V detected by a vehicle speed sensor 12and, then, the electric current to be supplied to the motor 20 iscontrolled based on the thus-computed steering assisting command valueI.

The control unit 30 is mainly constituted by a CPU. An ordinary functionto be executed by a program in the CPU is shown in FIG. 9.

The function and an operation of the control unit 30 is now described.The steering torque T to be inputted after detected by the torque sensor10 is phase-compensated by a phase-compensating device 31 for enhancingstability of a steering system and the thus-phase-compensated steeringtorque TA is inputted to a steering assisting command value computingdevice 32. Further, the vehicle speed V detected by the vehicle spedsensor 12 is also inputted to the steering assisting command valuecomputing device 32. The steering assisting command value computingdevice 32 determines the steering assisting command value I which is acontrol target value of the electric current to be supplied to the motor20 based on the inputted steering torque TA and the vehicle speed V. Thesteering assisting command value I is not only inputted to a subtractingdevice 30A but also supplied to a differential compensating device 34 ofa feed-forward system for enhancing a response speed and, then,deviation (I-i) of the subtracting device 30A is not only inputted to aproportional computing device 35 but also inputted to an integralcomputing device 36 of a feedback system for improving characteristicsthereof. The output from each of the differential compensating device 34and the integral computing device 36 is inputted to an adding device 30Bin an addition manner and, then, an electric control value E which is aresult of such addition in the adding device 30B is inputted to a motordrive circuit 37 as a motor drive signal. A motor electric current valuei of the motor 20 is detected by a motor electric current detectingcircuit 38 and, then, the thus-detected motor electric current value iis inputted to the subtracting device 30A, to thereby be fed back.

On the other hand, the mechanism as shown in FIG. 8 is shown in FIG. 10in terms of a transfer function. In FIG. 10, a block 301 is a transferfunction K(s) of the control unit 30, a block 201 is a transfer functionof the motor 20 which has characteristics of primary lag function, and ablock 202 indicates a torque coefficient K_(t) of the motor 20. A block3A is a gear ratio G_(r) of the reduction gear 3, and an output from thegear ratio G_(r) and a steering torque Th are inputted to a addingdevice 41 and, through an subtracting device 42, inputted to a transferfunction 501 of the pinion-rack mechanism 5. An angular speed ω which isan output from a transfer function 501 becomes an angle θ by passingthrough an integral factor 502 and, then, the angle θ is fed back to thesubtracting device 42 through a block 43 of dynamic characteristicsKv(s) of the vehicle. Further, the angle θ is inputted to a subtractingdevice 44 together with a steering wheel angle θ_(h) and, then, ansubtraction result thereof is inputted, via a spring coefficient(K_(tb)) 503 of a torsion bar, to an MAP 40 corresponding to thesteering assisting command value computing device 32 and, thereafter, anoutput from the MAP 40 is inputted to the control unit 301.

Frequency response characteristics of the control unit 301 is shown inFIG. 11, in which FIG. 11(A) shows gain characteristics, while FIG.11(B) shows phase characteristics. Further, torque characteristics ofthe torsion bar is shown in FIG. 12(A), while the angle is shown in FIG.12(B). These change in accordance with gains 1/150, 1, 10 and 50 of theMAP 40 as shown in (a), (b), (c) and (d), respectively. FIG. 12 showsresults of performing tuning as shown in FIG. 10 which show states ofangles at the time of changing the gain of MAP 40 of the feedback signalby 1/150, 1, 10, and 50. From these results, it is found that, sincethere is no substantial difference among results obtained by the gains1/150, 1, 10 and 50, it is difficult to perform tuning.

A conventional electric power steering apparatus is configured such thatit can simultaneously design stability of a system and a treatmentagainst road surface information and disturbance information by a robuststabilization compensating device. The robust stabilization compensatingdevice is a compensating device as described in, for example, JP-A No.8-290778 which has a characteristic formula represented byG(s)=(s²+a1·s+a2)/(s²+b1·s+b2) in which s represents a Laplace operator,removes a peak value of resonance frequency of a resonance systemcomprising an inertia factor and a spring factor contained in thesteering torque T and compensates a phase deviation of the resonancefrequency which inhibits the stability and response of the controlsystem.

However, it is difficult, from a standpoint of tuning, to treat aplurality of information and signals in a plurality of frequency bandsby a single compensating device. Particularly, when mechanical orelectric characteristics is changed even to a small extent, there is aproblem in that it takes longer time in tuning. Further, unless by afairly experienced engineer, there is a problem in that an apparatus ofsame performance can not be obtained.

The present invention has been accomplished under these circumstancesand an object of the present invention is to provide an electric powersteering apparatus which is easily tunable, constituted at a low costand can obtain a safe, comfortable steering feeling.

DISCLOSURE OF THE INVENTION

The present invention relates to an electric power steering apparatuswhich controls a motor that gives a steering assisting force to asteering mechanism based on an electric current controlling value whichis computed from a steering assisting command value which has beencomputed by a computing device based on a steering torque generated in asteering shaft and an electric current value of the motor and the statedobject of the present invention can be attained by being provided with aself-aligning torque estimating section which estimates a self-aligningtorque by a disturbance observer constitution and a steering torquefeedback section which performs definition of a steering reaction forcebased on a self-aligning torque estimated value which has been estimatedby the self-aligning torque estimating section and feeds the steeringreaction force back to the steering torque.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing an example of a constitution (transferfunction) of an electric power steering apparatus of feedback controlsystem using an SAT and a steering torque according to the presentinvention;

FIG. 2 is a view showing an example of a frequency response of a controlunit;

FIG. 3 is a diagram showing an example of frequency characteristics ofan SAT estimating section;

FIG. 4 is a view showing an example of characteristics of a staticcharacteristic sub-section of a feeling characteristic section;

FIG. 5 is a view showing an example of characteristics of a dynamiccharacteristic sub-section of a feeling characteristic section;

FIG. 6 is a view explaining an effect according to the presentinvention;

FIG. 7 is a view explaining an effect according to the presentinvention;

FIG. 8 is a view showing a mechanism of an ordinary electric powersteering apparatus;

FIG. 9 is a block diagram showing an example of a constitution of acontrol unit of an electric power steering apparatus;

FIG. 10 is a block diagram showing a transfer function system of thepower steering apparatus as shown in FIG. 8;

FIG. 11 is a view showing frequency characteristics of a conventionalcontrol unit; and

FIG. 12 is a view showing conventional torsion bar characteristics.

BEST MODE FOR CARRYING OUT THE INVENTION

According to the present invention, a self-aligning torque estimatingsection for estimating a self-aligning torque by a disturbance observerconstitution is provided and, then, definition of a steering reactionforce is performed based on a self-aligning torque estimated value whichhas been estimated by the self-aligning torque estimating section and amotor rotation (angle) signal or angular speed signal and, thereafter,the steering reaction force is fed back to a steering torque. Further,according to the present invention, the self-aligning torque isestimated and the resultant self-aligning torque estimated value is fedback to the steering torque together with torque information of atorsion bar. A control unit according to the present invention has arobust property in that stability of a system can be secured regardlessof fluctuations of characteristics (for example, resonance frequency) ofthe system. Still further, the definition of static characteristics ofthe steering reaction force is determined based on a necessary steeringforce and the self-aligning torque estimated value and the definition ofdynamic characteristics of the steering reaction force is performed suchthat a gain of transfer function in a frequency band of informationwhich is desirous to be conveyed to a driver is allowed to be largewhile the gain of transfer function in the frequency band of informationwhich is not desirous to be conveyed to the driver is allowed to besmall. For this account, the definition of necessary steering reactionforce can easily be performed and a low-cost constitution and a stable,comfortable steering feeling can be realized.

Furthermore, since a motor rotation angle signal (or a motor angularspeed signal) and a motor electric current command value are allowed tobe used for estimating the self-aligning torque, a constitution of theself-aligning torque estimating section of high precision and low-costcan be realized by using the motor rotation angle signal (or the motorangular speed signal) used for controlling the motor.

FIG. 1 shows an example of a constitution in a block diagram accordingto the present invention, in which a steering torque Th is inputted in acontrol unit 100 (transfer function: K(s)) and, then, a motor electriccurrent command value Ir which is an output therefrom is inputted in anadding device 105 via a motor 102 (transfer function: 1/(T1·s+1) offirst order lag function, a torque function 103 (transfer function:K_(t)) of the motor and a gear ratio 104 (transfer function: G_(r)) of areduction gear. The addition result of the adding device 105 is inputtedin a pinion-rack mechanism 130 (transfer function: 1/(J_(pt)·s+C_(pt))via a subtracting device 106. A motor angular speed ω which is an outputfrom the pinion-rack mechanism 130 is converted into an angle θ bypassing through an integral factor 131 and the thus-converted angle θ isfed back to the subtracting device 106 via dynamic characteristics 132(transfer function: Kv(s)) of a vehicle. The Jpt of the pinion-rackmechanism 130 is a pinion-base inertial moment, while the Cpt thereof isa pinion-base damping coefficient. Further, the angle θ is inputted to asubtracting device 133 together with a steering wheel angle θ_(h) and,then, the subtraction result therein is inputted in an adding device 135via a spring coefficient 134 (transfer function: K_(tb)) of the torsionbar, while a self-aligning torque estimated value ES is also inputted tothe adding device 135 from a self-aligning torque estimating section110. The self-aligning torque estimating section 110 performs anestimation of a self-aligning torque (SAT) from the motor electriccurrent command value Ir and the motor angular speed ω, and a steeringtorque feedback section 120 which performs a definition of a steeringreaction force (complementary component) AT based on the self-aligningtorque estimated value ES which has been estimated by the self-aligningtorque estimating section 110 and feeds the steering reaction force backto the steering torque Th via a subtracting device 101 is provided.

The self-aligning torque estimating section 110 comprises a factor 111(Q/Pn) in which the motor angular speed ω is inputted and treated and afactor 112 (M·Q) in which the motor electric current command value Irisinputted and treated, is allowed to determine a deviation between theoutput of the factor 111 and the output of the factor 112 by asubtracting device 113 and, then, outputs the result(the deviation) asthe self-aligning torque estimated value ES. Q(s) indicates a low-passfilter, while Pn(s) indicates a theoretical model of rack-pinion. Thefactor 111 is constituted by a transfer function Q(s) and a transferfunction Pn⁻¹, while M indicates a transfer function (=1/(T1·s+1))which, then, forms the factor 112 by being multiplied by Q(s).

The M·Q of the factor 112 is a product of an electric characteristic Mof the motor and a low-pass filter Q, while the Q/Pn of the factor 111is a quotient obtained by dividing the low-pass filter Q by an idealmodel Pn. The basis on which the self-aligning torque estimating section110 can compute the self-aligning estimated torque value ES is asdescribed below. A torque Tm is represented by the following formula(1):Tm=M(s)×Ir   (1)wherein M(s)=(Kt×Gr)/(T1·s+1).

Further, the motor angular speed ω is represented by the followingformula (2):ω=P(s)×[Tm+Ttb−SAT]  (2)wherein P(s)=1/(Jpt·s+Cpt)

Still further, based on a constitution of the self-aligning torqueestimating section 110, the self-aligning estimated value ES isrepresented by the following formula (3):ES=M·Q−Q/Pn   (3)

Therefore, when the formula (3) is substituted by the formulae (1) and(2), the result comes to be as follows: $\begin{matrix}\begin{matrix}{{ES} = {{{Q(s)} \times {Tm}} - {{Q(s)} \times {P(s)} \times {\left\lbrack {{Tm} + {Ttb} - {SAT}} \right\rbrack/{{Pn}(s)}}}}} \\{= {\left\{ {{{Q(s)} \times {Tm}} - {{Q(s)} \times \left\lbrack {{{P(s)}/{{Pn}(s)}} \times {Tm}} \right\rbrack}} \right\} \times}} \\{\left\{ {{Q(s)} \times \left\lbrack {{P(s)}/{{Pn}(s)}} \right\rbrack \times \left\lbrack {{SAT} - {Ttb}} \right\rbrack} \right\}}\end{matrix} & (4)\end{matrix}$

Further, when the pinion-base inertial moment Jpt and pinion-basedamping coefficient Cpt value of the pinion-rack mechanism 130 aredetermined such that the relation of Pn(s)=P(s) is held, the followingrelation can be obtained:ES=Q(s)×[SAT−Ttb]  (5)

Therefore, since the addition result of the adding device 135 isinputted in the steering feedback section 120, the following formula (6)can be obtained: $\begin{matrix}\begin{matrix}{{SatE} = {{ES} + {Ttb}}} \\{= {{{Q(s)} \times {SAT}} + {\left\lbrack {1 - {Q(s)}} \right\rbrack \times {Ttb}}}}\end{matrix} & (6)\end{matrix}$

Therefore, in the range in which Q(s)=1, the following formula can beobtained:SatE=SAT   (7)

From the above description, the relation between the self-aligningtorque SAT and the self-aligning torque estimated value ES isrepresented by the formula (5), while a relation between theself-aligning torque SAT and the addition result SatE is represented bythe formula (7).

Further, the filter Q, the motor characteristics M and characteristicsof the model Pn can be represented by respective formulae as describedbelow.

When the filter Q(s) uses the angular speed ω and, also, Tq is a timeconstant, the Q(s) is represented by the following formula:Q(s)=1/(Tq·s+1)   (8)

When it uses the angle θ and, also, b0 and b1 are each a constant, theQ(s) is represented by the following formula:Q(s)=b1/(s ² +b0·s+b1)   (9)

Each of them (8) (9) shows a high-cutoff filter. Further, the motorcharacteristics M(s) and model P(s) can be represented by the followingformulae:M(s)=Kt×Gr/(T1·s+1)   (10)P(s)=1/(Jpn·s+Cpn)   (11)

As described above, the addition result SatE is inputted to the steeringfeedback section 120, while the deviation (AT−Th) between the steeringtorque Th and the steering reaction force AT which is an output from thesteering feedback section 120 are inputted to the control unit 100 and,in the present control system, the steering torque Th and the SATinformation are utilized for the feedback control.

Further, according to the present invention, characteristics of thecontrol unit 100 are allowed to be gain and phase characteristics asshown in FIG. 2 without containing an integral factor and, accordingly,they become a proportional factor in a low frequency range and cutoffcharacteristics in a high frequency range. Characteristics of theself-aligning torque estimating section 110 are allowed to be those asshown in FIG. 3. In FIG. 3, the actual self-aligning torque SAT (solidline) and the estimated self-aligning torque ES (broken line) are shown.Further, the steering torque feedback section 120 comprises a dynamiccharacteristic sub-section 121 and a static characteristic sub-section122. The dynamic constituting sub-section 121 has characteristics asshown in FIG. 4, while the static characteristic sub-section 122 hascharacteristics as shown in FIG. 5. The static characteristicsub-section 122 has a function of a feeling characteristic section suchthat it gives a complementary effect to a torque which a driver feelsand, in the present example, is separated into a function block ofshowing a gain g and a function block of showing a curve pattern. InFIG. 4, a range AR2 (angular frequency from ω₁ to ω₂) indicates afrequency band of information which is desirous to be conveyed to adriver, while ranges AR1 (angular frequency of ω₁ or less) and AR3(angular frequency of ω₂ or more) each indicate a frequency band ofdisturbance information which is desirous to be suppressed. AlthoughFIG. 5 shows static characteristics to be targeted, the gain g isactually fluctuated in an appropriate range (1/150, 1, 10 and 50) so asto cover characteristics as shown in FIG. 5.

In a constitution as described above, the deviation (AT−Th) between thesteering torque Th and the steering reaction force AT which is an outputfrom the steering torque feedback section 120 is obtained by thesubtracting device 101 and the deviation (AT−Th) is inputted in thecontrol unit 100 and, then, the motor electric current command value Irwhich is an output therefrom not only drives the motor 102 but also isinputted in the self-aligning torque estimating section 110 of adisturbance observer constitution. The control unit 100 compensatesstability of an entire system and has robust characteristics by securingthe stability of the entire system regardless of fluctuations ofcharacteristics (for example, resonance frequency) of the system.Determination of the transfer function K(s) of the control unit 100 mayeither be performed by PID or a try-and-error method.

An output of the motor 102 is inputted in the adding device 105 via themotor torque coefficient 103 (Kt) and the gear ratio 104 (Gr) and, then,the resultant addition value is inputted in the pinion-rack mechanism130 (1/(Jpt·s+Cpt)) via the subtracting device 106. An output from thepinion-rack mechanism 130 is inputted in the subtracting device 133 viathe integral factor 131 (1/s) and the output of the integral factor 131is inputted in the factor 132 which indicates dynamic characteristics ofthe vehicle and, then, the self-aligning torque SAT which is an outputtherefrom is inputted in the subtracting device 106. Further, theaddition result in the subtracting device 133 is outputted via thespring coefficient 134 (Ktb) of the torsion bar.

An output Ttb from the spring coefficient 134 (Ktb) is not only inputtedin the adding device 135 but also fed back to the adding device 105,while the motor angular speed ω which is an output from the pinion-rackmechanism 130 is inputted in the self-aligning torque estimating section110. Then, the self-aligning torque estimated value ES from theself-aligning estimating section 110 is inputted in the steering torquefeedback section 120 via the adding device 135. The steering torquefeedback section 120 comprises the dynamic characteristic sub-section121 and the static characteristic sub-section 122 of feelingcharacteristics of torque which a human being feels.

According to the present invention, the self-aligning torque for theelectric power steering and the feedback control system using thesteering torque are utilized and the gist thereof, that is, the controlunit 100 of the feedback, being characterized by the frequencycharacteristics (gain and phase) in FIG. 2, has no integral factor buthas proportional characteristics in a low frequency range and cutoffcharacteristics in a high frequency range. The steering torque Th ismeasured by a torque sensor of the torsion bar, while the self-aligningtorque SAT is not measured but is estimated by the self-aligning torqueestimating section 110 of the observer constitution. The thus-estimatedself-aligning torque ES and the measured self-aligning torque SAT cometo be those as shown in FIG. 3.

The result of the characteristics K(s) of the control unit 100 appliedto the case of FIG. 2 come to be those as shown in FIG. 6; the featurethereof is favorable. It is found that, compared with thecharacteristics indicating the result of the conventional apparatus, thedifference caused by the change of the gain comes to be large and,accordingly, tuning is easily performed. By contrast, when thecharacteristics K(s) of the control unit 100 is applied to the case ofFIG. 3, the results come to be those as shown in FIG. 7; it is foundthat the feature thereof is unfavorable. Namely, in FIG. 7, when thegain g is changed, the torque of the torsion bar is changed but thepinion angle is largely changed and, accordingly, a following responseproperty of the steering is deteriorated and it also becomes difficultto perform tuning. In FIG. 6, when the gain g is changed, the torque ofthe torsion bar is changed in a regular manner and, moreover, since thepinion angle is not largely changed, the following response property ofthe steering is not deteriorated and, accordingly, it is easy to performtuning.

In the aforementioned example, the angular speed ω is used for theself-aligning torque estimation. However, it is also possible to performestimation thereof by using the angle.

Industrial Applicability

According to a power steering apparatus of an automotive vehicleaccording to the present invention, treatments of road surfaceinformation, disturbance information and the like and designing ofsteering stability can independently be designed, to thereby beingcapable of providing a low-cost constitution, easy tuning, and a stable,comfortable steering feeling.

1. An electric power steering apparatus, which controls a motor thatgives a steering assisting force to a steering mechanism based on anelectric current controlling value which is computed from a steeringassisting command value which has been computed by a computing devicebased on a steering torque generated in a steering shaft and an electriccurrent value of the motor, being characterized by comprising aself-aligning torque estimating section which estimates a self-aligningtorque and a steering torque feedback section which performs definitionof a steering reaction force based on a self-aligning torque estimatedvalue which has been estimated by the self-aligning torque estimatingsection and feeds the steering reaction force back to the steeringtorque.
 2. The electric power steering apparatus as set forth in claim1, wherein said self-aligning torque estimating section estimates saidself-aligning torque by a disturbance observer constitution.
 3. Theelectric power steering apparatus as set forth in claim 1, wherein theself-aligning torque estimating section is allowed to estimate theself-aligning torque from a motor rotation signal or angular speedsignal and a motor electric current command value.
 4. The electric powersteering apparatus as set forth in claim 1, wherein definition of staticcharacteristics of the steering torque feedback section is determinedbased on the steering reaction force and the self-aligning torqueestimated value.
 5. The electric power steering apparatus as set forthin claim 1, wherein the definition of dynamic characteristics of thesteering reaction force of the steering torque feedback section isperformed such that a gain of a transfer function in a frequency band ofinformation which is desirous to be conveyed to a driver is allowed tobe large, while the gain of the transfer function in the frequency bandof information which is not desirous to be conveyed to the driver isallowed to be small.
 6. The electric power steering apparatus as setforth in claim 1, wherein a characteristic of a controller into which adeviation between the steering torque and an output from the steeringtorque feedback section is inputted is allowed to be a proportionalfactor in a low range and a cutoff factor in a high range, withoutcontaining an integral factor.